1. Field of the Invention
This invention relates to a forehearth and more particularly relates to a feeder forehearth for supplying glass from a furnace to glass forming machinery.
2. Description of Related Art
A forehearth is a channel linking a glass forming machine to the glass melting furnace. In addition to providing a physical link, its most important function is to thermally condition the glass to match the requirements of the forming machine. The normal requirement is that the forehearth should take hotter glass from the furnace and deliver cooler glass to the forming machine in such a manner that no temperature gradients exist within the glass as supplied to the forming machine. In reality this level of glass temperature homogeneity is virtually impossible to achieve. However new processes and faster production speeds make the importance of temperature homogeneity considerably more important.
To obtain the necessary lower temperatures required by the forming machines some form of cooling must be provided. To ensure temperature homogeneity, a heat input is also required to control the loss of temperature particularly in respect of areas more subject to conductive heat loss, e.g. from the side walls of the forehearth.
One current forehearth proposal employs longitudinal hot face forced convection cooling. In this method of cooling, cooling air is introduced to the forehearth via a roof block arrangement and exits from an exhaust located in the centre of the forehearth at a position further down stream from the inlet. The amount of heat removed from the forehearth is then a function of the air flow rate, the residence time of the cooling air in the forehearth, and various physical parameters such as the heat transfer co-efficient and the emissivity of the roof block. In this proposal the shape of the roof block is such as to partially divide the forehearth into three chambers longitudinally. The outer two chambers concentrate combustion gases from side wall mounted burners to the sides of the forehearth (which suffer more heat loss) and the central chamber is used to channel cooling air from the air inlet to the central exhaust. In this construction no other exhaust flue exists in the cooling section of the forehearth. Control of the forehearth is such that if the temperature of the glass (as measured by a suitable temperature sensor) is seen to be over a given set point then a controller sends a signal which restricts the amount of combustion gases entering the burners and at the same time increases the amount of cooling air entering the forehearth central chamber. A damper fitted above the central cooling air exhaust lifts to allow the extra cooling air to be removed. Since no separate flues are provided for the combustion gases, these also exhaust through the cooling gas outlet.
The present invention seeks to provide a forehearth having improved temperature control and homogeneity and in which the combustion gases and cooling gases are as far as possible kept separate.